Heretofore, heat-shrinkable resin films have been widely used in heat shrinkable applications such as shrink packaging, and shrink labels. In particular, oriented films of polyvinyl chloride resin, polystyrene resin, polyester resin, and the like have been used for labels, cap seals, or integrated packaging for a variety of containers such as polyethylene terephthalate (PET) containers, polyethylene containers, and glass containers.
After producing such a heat-shrinkable film, it is subjected to a printing step of various types of patterns. After printing, if necessary, the film is slit into pieces in the desired size depending on the size of a label to be used for a particular end product. Then, both right and left ends of the film are superposed on each other and bonded by means of solvent welding or the like so that the film is formed into a tube. The tube is cut into label pieces, bag pieces, or the like. Each label or bag piece is opened and put on a container and then transported on a belt conveyor or the like through a steam-blowing type heat-shrinking tunnel (steam tunnel) or a hot air blowing type heat-shrinking tunnel (hot air tunnel) to tightly fit the label or bag to the container by heat shrinkage.
In the above process, if the ink has inferior ink adhesion to the film in the printing step, dropping off or peeling off of the ink occurs to deteriorate the commercial value of a product. In particular, in recent years, alkali-removable type ink which is removable in an aqueous alkaline solution, aqueous type ink with reducing or without using an environmentally-harmful organic solvent, etc. have been developed for the purpose of recycling PET bottle labels and the like. Sometimes, however, properties of such types of ink, for example, ink adhesion to a film and the like are lowered in comparison with those of conventional type ink due to achievement of the given desired characteristics. Then, there has been a demand for a heat-shrinkable film which is excellent in adhesion for various types of ink, can exhibits excellent removable properties in an aqueous alkaline solution, and is excellent in opening properties after cut the film into label pieces as described above.
For improving ink adhesion for the above-described various types of ink, for example, it is considered to subject the surface of a film to surface treatment such as normal corona treatment under an air atmosphere so as to increase wet tension of the surface of the film. However, when wet surface tension of a film is increased by such surface treatment, the film causes a problem of fusion or blocking at a cut portion at the time of cutting the film into label pieces and therefore causes opening failure in a label fitting step. This further causes such a problem that, when such a film is shaped into a tube by solvent welding with tetrahydrofuran and 1,3-dioxolane, the solvent resistance of the film is lowered and the solvent-welded portion loses planarity to change its shape into a so-called sea weed-like shape, or the solvent-welded portion of the tube causes blocking together with another portion of the film.